[PATCH 3.18-rc3 v3 2/2] trace: kdb: Fix kernel livelock with empty buffers

From: Daniel Thompson
Date: Fri Nov 14 2014 - 06:22:12 EST


Currently kdb's ftdump command will livelock by constantly printk'ing
the empty string at KERN_EMERG level if it run when the ftrace system is
not in use. This occurs because trace_empty() never returns false when
the ring buffers are left at the start of a non-consuming read [launched
by ring_buffer_read_start()].

This patch changes the loop exit condition to use the result of
trace_find_next_entry_inc(). Effectively this switches the non-consuming
kdb dumper to follow the approach of the non-consuming userspace
interface [s_next()] rather than the consuming ftrace_dump().

Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
Cc: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/trace/trace_kdb.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index 8faa7ce58814..b0b1c44e923a 100644
--- a/kernel/trace/trace_kdb.c
+++ b/kernel/trace/trace_kdb.c
@@ -59,19 +59,19 @@ static void ftrace_dump_buf(int skip_lines, long cpu_file)
ring_buffer_read_start(iter.buffer_iter[cpu_file]);
tracing_iter_reset(&iter, cpu_file);
}
- if (!trace_empty(&iter))
- trace_find_next_entry_inc(&iter);
- while (!trace_empty(&iter)) {
+
+ while (trace_find_next_entry_inc(&iter)) {
if (!cnt)
kdb_printf("---------------------------------\n");
cnt++;

- if (trace_find_next_entry_inc(&iter) != NULL && !skip_lines)
+ if (!skip_lines) {
print_trace_line(&iter);
- if (!skip_lines)
trace_printk_seq(&iter.seq);
- else
+ } else {
skip_lines--;
+ }
+
if (KDB_FLAG(CMD_INTERRUPT))
goto out;
}
--
1.9.3

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